1. Field of the Invention
The present invention relates to a zoom lens system and in particular to a high zoom ratio zoom lens system capable of shifting an image.
2. Related Background Art
An optical system capable of moving (shifting) an image perpendicularly to the optical axis by moving (shifting) one or some of lens elements constructing the optical system substantially perpendicularly to the optical axis has been known. As for such optical systems, a zoom lens system capable of shifting an image by shifting one or some of lens elements provided in the zoom lens system has been proposed (see, for example, Japanese Patent Application Laid-Open No. 2003-140048 and Japanese Patent Application Laid-Open No. 2-081020).
In the present specification, one or some of lens elements being shifted substantially perpendicularly to the optical axis is hereinafter called a shift lens group.
Recently, a zoom lens has widely used as a photographic lens. When a zoom lens is used as a photographic lens, it makes you possible to take a photograph closer to the subject, so it has a merit that you can take a photograph just as you intend. According to popularization of a zoom lens as a photographic lens, a high zoom ratio zoom lens capable of shooting closer to the subject has come onto the market.
As a high zoom ratio zoom lens capable of shooting closer to the subject, a positive-negative-positive-positive four-lens-group type zoom lens has been known (see, for example, Japanese Patent Application Laid-Open No. 2001-117005 and Japanese Patent Application Laid-Open No. 11-142739).
A positive-negative-positive-positive type zoom lens is composed of, in order from the object, a first lens group having positive refractive power, a second lens group having negative refractive power, a third lens group having positive refractive power, and a fourth lens group having positive refractive power. When the state of lens group positions varies from a wide-angle end state (which gives the shortest focal length) to a telephoto end state (which gives the longest focal length), at least the first lens group and the fourth lens group move to the object side such that a distance between the first lens group and the second lens group increases, a distance between the second lens group and the third lens group decreases, and a distance between the third lens group and the fourth lens group decreases.
According to further popularization of a zoom lens as a photographic lens, in order to meet user's expectation to improve portability, compact and lightweight zoom lenses have been proposed.
On the other hand, in particular for a compact and lightweight zoom lens, an image tends to be blurred during exposure by minute vibration produced on a camera while shooting such as a camera shake caused by a photographer upon releasing a shutter button. When the amount of the camera vibration is assumed to be constant, the amount of image blurring increases in accordance with the increase in the focal length of the lens, so the minute camera vibration causes severe deterioration on the image.
Accordingly, a method for compensating the above-described image blur caused by the camera shake by combining a zoom lens capable of shifting an image with a driver, a detector and a controller has been known (see, for example, Japanese Patent Application Laid-Open No. 10-282413). In such zoom lens, the detector detects a camera shake. The controller controls the shift lens group giving the driver a driving amount in order to correct the shake detected by the detector. The driver corrects the image blur caused by the camera shake by driving the shift lens group substantially perpendicularly to the optical axis.
Generally, in a zoom lens, it is necessary to correct various aberrations for each lens group to obtain given optical performance as a whole zoom lens. The state of aberration correction required to each lens group has a certain range, and the range generally becomes narrow when the zoom ratio becomes large.
On the other hand, in an optical system capable of shifting an image, in order to suppress variation in various aberrations produced upon shifting an image, there is a state of aberration correction required for the shift lens group only.
Accordingly, the state of aberration correction required for the shift lens group in order to obtain good optical performance when the zoom ratio becomes large is completely different from that required for the shift lens group in order to correct aberrations produced upon shifting an image to obtain good optical performance. Therefore, it is very difficult to combine to attain a high zoom ratio and to construct an optical system capable of shifting an image.
A conventional zoom lens having vibration reduction correction disclosed in Japanese Patent Application Laid-Open No. 2003-140048, however, has a large number of lens elements, and a vibration reduction mechanism has to be put into the lens barrel. Accordingly, the total lens length and the diameter of the lens barrel become large, so the compactness tends to be spoiled. Moreover, when the zoom lens is made to be a high zoom ratio with having a vibration reduction correction, deterioration in optical performance is severe, so that it becomes difficult to maintain sufficient optical performance as a zoom lens.
A zoom lens disclosed in Japanese Patent Application Laid-Open No. 10-282413 has a large number of lens elements, so that when the state of lens group positions varies from the wide-angle end state to the telephoto end state, degree of freedom for selecting zoom trajectory of each lens group is large. Accordingly, high optical performance can be obtained. However, the driving mechanism for moving each lens group becomes complicated and the factors to produce mutual decentering of each lens group upon manufacturing increase, so that it becomes difficult to secure stable optical performance.